Underbalanced drilling has many advantages. In some cases, oil/gas well flow during underbalanced drilling of a well has been sufficient to pay for the cost of drilling the well even prior to completion of the well. Other advantages include that of avoiding formation damage for a better performing well and more accurate logging measurements of the well contents. For a discussion of advantages of underbalanced drilling including methods of controlling the well using an exemplary rotating blow out preventer, please refer to U.S. Provisional Application Ser. No. 60/083436, entitled ROTATING BOP AND METHOD, filed Apr. 29, 1998, to Hosie et. al, subsequently filed as U.S. patent application Ser. No. 09/178,006, filed Oct. 23, 1998, which applications are hereby incorporated herein by reference. Thus, there are numerous and significant advantages for underbalanced drilling of a well.
However, various problems exist for underbalanced drilling. These problems relate mainly to controlling the well in certain circumstances. A possible problem that may typically arise is the need to remove the drill string from the well. There are many common reasons that the drill string must be removed from the well prior to completion of drilling. It may be necessary to remove the drill string for reasons such as the need to change out the drill bit, steering tool, mud motor, and the like. Although pressure is controlled at the surface, such as with a rotating BOP, the weight of the drill string holds the drill string within the well bore. The string light point is the depth or position where the upward forces acting on the drill string become greater than the downward forces. Many factors are involved and may vary as to exactly where in a well the drill string becomes string light. Such factors include but are not necessarily limited to, or may not necessarily always include, the following: surface pressure, down hole pressure, flow rate, hole size, drill pipe size and weight, the amount of drill pipe in the hole, bit size, casing size, and fluid or gas properties. Generally assuming other factors remain constant, as the drill string is pulled further out of the hole, the downwardly acting forces decrease due to decreased drilling string weight.
If an attempt to remove the drill string is made with pressure at the surface, then at some point as the drill string is being removed the pressure may begin to push or accelerate the drill string out of the well bore. This is a dangerous situation that could conceivably lead to a blow out. Once the pipe begins to move upwardly there may be developed a significant momentum such that the blow out preventers may not be able to stop the upward movement. Once the heavy pipe string is moving upwardly, closing the rams may result in tearing the rams out rather than stopping the upward movement of the pipe. In this case, the rams will not be available to shut in the well after the pipe has been pushed from the well bore, assuming there is someone left at the rig site to activate the rams after the drill pipe is ejected from the well. The forces are great enough so that ejected drill pipe may be found quite far from the rig. As well, sparks produced can ignite gas to produce a hot fire that can melt a drilling rig within minutes. Blow outs can result in costly problems such as personnel injury, damage to the drilling rig, environmental damage, and loss of the hole. Presently, methods used to avoid a blow out situation are effective but have significant disadvantages.
While it may be possible to bleed off the surface pressure prior to reaching the point where the string becomes "string light" and begins to move upwardly, this practice is risky. For instance, a bridge in the bore hole may form in the formation that temporarily permits a bleed off to appear to occur. If the bridge should break at the wrong moment with the pipe nearly out of the hole, then significant formation pressure may be applied at the surface to result in a blow out.
A very effective and safe practice is to kill the well prior to removal of the drilling string. However, this practice is undesirable because the advantages of underbalanced drilling may then be lost. Once the drill string is lowered back into the well bore below the string light point it may be possible to adjust the drilling fluids so that underbalanced drilling continues. However, formation damage may have already occurred that is substantially or partially irreversible.
Another very effective and safe practice is that of providing a snubbing unit for removing the drilling string. However, the snubbing unit takes considerable time to rig up, requires considerable additional time while tripping the well, and then requires considerable additional time to rig down. Thus, the cost of tripping the drill string can be quite considerable due to the rig time costs and snubbing unit costs. Additional tripping of the well may also be necessary and again require the snubbing unit. This procedure then, while effective and safe, increases drilling costs considerably.
Consequently, an improved method and apparatus is desirable for removing drill string from a well bore that is drilled underbalanced. Such an improved method and apparatus should provide for quick, but safe, removal of the drill string from the well without the need to kill the well. The method and apparatus should be useful for repeated tripping of the drill string whenever necessary without significant time and cost increases. Those skilled in the art will appreciate the present invention that addresses these and other problems.